Electrowetting devices, and in particular electrowetting lenses, are known in the art, and generally comprise a refractive interface between first and second immiscible liquids that is movable by electrowetting.
FIG. 1 represents FIG. 4 of European Patent Application EP 1662276, and illustrates a variable focus lens 10 according to one example of the prior art. Lens 10 comprises two transparent windows 12, 14, arranged in parallel and facing each other, and delimiting, in part, an internal volume 15 containing two immiscible liquids 16, 18, with different optical indices. Where the two liquids meet they form an optical interface 19 in the form of a meniscus, which can have a number of different shapes, examples being shown by lines A and B. The liquids 16, 18 have substantially equal densities, and one is preferably an insulating liquid, for example comprising oil and/or an oily substance, and the other is preferably a conductive liquid comprising for example an aqueous solution.
Windows 12, 14 are preferably transparent plates formed of an optical transparent material such as glass.
The structure of the lens 10 that contains the liquids comprises a cap 20 to which transparent window 12 is fixed, for example by glue 21, and a body 22 to which transparent window 14 is fixed, for example by glue 23. The cap 20 and body 22 are separated by a gasket 24. Cap 20 comprises a substantially cylindrical side wall 26, while body 22 comprises a substantially cylindrical side wall 27, and gasket 24 is positioned between the side walls 26 and 27, to ensure the tightness of the lens structure.
The cap 20 and body 22 in this example form electrodes of the lens. Cap 20 is used to provide a voltage to the conductive liquid 18. Body 22 is used to provide a voltage close to the edge of the liquid-liquid interface 19. The edge of the liquid-liquid interface 19 contacts an insulated conical surface 26 of the annular body. The parts of body 22 in contact with the liquids 16, 18, including conical surface 26, are coated with an insulating layer (not shown). The interface 19 traversed an opening in the annual body 22 through which light rays can pass.
Due to the electrowetting effect, it is possible, by applying a voltage between the cap 20 and the body 22, to change the positioning of the edge of the liquid-liquid interface on the conical surface 26, and thereby change the curvature of the refractive interface between liquids 16 and 18. For example, the interface may be changed from initial concave shape shown by dashed line A, to a convex shape as shown by solid line B. In this way, rays of light passing through the lens perpendicular to windows 12, 14 in the region of the refractive interface A, B will be focused more or less depending on the voltage applied between the cap 20 and body 22.
Body 22 is formed of a metal, machined into the shape shown in FIG. 1. For example, as body 22 is annular, it can be turned in a machine, and blades used to cut the required form. There is a general desire to make liquid lenses thinner, especially for use in some industries such as the mobile telephone industry. This has led the present applicant to design lenses having bodies of very small height, inferior to 0.5 mm. However, it has been found that for such small dimensions, machining the body is inaccurate. This inaccuracy necessitates an increase in the margins needed for certain surfaces of the body, which leads to a decrease in the zone available for contact with the liquid interface, which in turn leads to a reduced focal range of the lens.